Telescopic sights for rifles typically include eyepiece and objective lenses positioned at opposite ends of a tubular housing. Conventional variable optical power telescopic sights also include an erector lens positioned medially of the eyepiece and objective lenses movable along a central longitudinal axis of the housing for adjustment of the optical power.
In these sights, an image focus adjustment compensating for variations in the distance from the marksman to the target is typically accomplished by longitudinal movement of the objective lens. One prevalent image focus adjustment implementation uses a threaded mounting ring that is coupled to the housing for rotation about the objective end of the housing. The marksman rotates the mounting ring, to adjust the position of a focusing element of the objective lens along the longitudinal axis of the housing.
A technique for varying the optical power of a telescopic sight is described in U.S. Pat. No. 6,351,907 to Otteman. This telescopic sight has a rotatable cam hub including a drive face and a spiral cam track formed in the drive face around an axis of rotation. An actuator slide of the adjustment mechanism is slidably mounted to a housing of the telescopic rifle sight for movement along a longitudinal axis of the housing. The actuator slide includes a cam follower that is operably engaged in the spiral cam track so that the actuator slide moves generally along the longitudinal axis in response to rotation of the cam hub to changing a setting of the adjustment mechanism. The actuator slide is operatively connected to a movable optical element positioned within the housing of the telescopic rifle sight to drive the optical element in response to rotation of the cam hub. The rate of linear movement of the optical element relative to the rate of rotation of the cam hub is controlled by the shape and arcuate angle of the spiral cam track. A spring biases the actuator slide relative to the housing to prevent rifle recoil from inducing changes to the setting of the adjustment mechanism
However, such telescopic sights of do not allow for both course and fine focus adjustment.